September 2007World Premiere!
Nomad Audio Sentinel
An open baffle that may be head and shoulders above the vast majority box loudspeakers.Review By Scott FallerClick here to e-mail reviewer

For this article, Iím stepping
away from my (loosely) self-imposed writing guidelines. After a lot of soul
searching, have found the things that trip my trigger most often have gain
stages that glow, have low power or high efficiency. These are the things in my
audio life that get my juices flowing. There is no point in trying to fool
myself (or anybody else) about this because if I did, Iíd have a miserable
time trying to write the articles. Sorry, thatís just part of my personal
bias. Iíve tried and tried writing about gear that isnít in my Ďsweet
spotí, but ultimately think Iíd rather be lashed to chair and forced to
listen to Diana Krall on a shitty CD player stuck on repeat, than write about
something that doesnít turn me on. Itís absolute torture.

So why exactly have I chosen to write about a speaker that is
84dB/W/m (on a good day) and has a crossover in it? Well, let me tell you. A few
months ago Paul Hilgeman decided to make the relatively short trek south to St
Louis and perform a demonstration to our local audio guys, the Gateway Audio
Society. Paulís company, Nomad Audio, is a speaker manufacturer whose base of
operations is in the greater Chicagoland area.

Paul brought a pair of his larger speakers, the Roninís. The
Ronin consisted of a sealed cabinet, which housed a powered woofer and an open
baffle midrange driver that has a concentrically loaded, a tweeter for the
highs. That evening about fifteen of us were treated to one of the better
sounding, low efficiency speakers Iíve (personally) had the pleasure of
listening to. While Paul was there we tried a couple of different types of
amplification, tubes (ala VTL) and one of the big Niagara Class D amps. As you
might imagine, I preferred the VTLís to the Class D but that (again) boils
down to my personal preference.

As I intently listened to Paulís creation, I couldnít help
but think to myself that he is really onto something here. These speakers
had tons of good stuff going on for them, huge soundstage, great depth and
imaging like Maggieís Ö.. but they had tremendous bass. Not that Maggieís
donít do bass (because they do), these just had significantly more grunt at
high SPLís. Needless to say, I was extremely impressed with what Iíd
experienced. Impressed enough that when Paul contacted me and let me know he was
releasing a Ďsmallerí, less expensive version of the Roninís, I jumped on
the chance to write about them.

The Design

When you look at the Sentinel, you can see it is a direct
descendant of its big brother the Ronin. They share the same open baffle
midrange and sealed woofer design. There are several differences though. The
Seas driver used as a midrange in the Ronin is now used as a woofer in the
Sentinel. Replacing the Seas Excel W22EX series for the midrange duties is the
T18REX series of coaxial drivers. This uses a 26mm soft dome tweeter, the same
as the Ronin.

The Sentinel comes standard finished in a light Bamboo. Paul
does offer several other wood finish options for a slight up charge. The back
side of the sealed woofer cabinet is sloped presumably to help knock down the
internal reflections and buildups but also to give a slightly larger surface to
give the midrange a Ďliftí in its open baffle arrangement. On the backside
of the Sentinel is a single pair of nice quality, gold plated binding posts.

Inside, fixed to the bottom of the speaker cabinet you will
find the crossover. Here Nomad Audio has chosen a combination of air core and
iron core chokes along with nice sounding pair of metalized polypropylene
capacitors. The style of chokes (air or iron core) is carefully chosen to avoid
saturation at the loudspeakerís power limits.

Paulís chosen crossover points are 400Hz for the woofer to
midrange and 2000Hz from the midrange to the tweeter. The slopes are 3rd and 2nd
Order respectively. Nomad Audio does offer impedance compensation (a Zobel
network) as an option to the speakers in case you are using a tube amp. The
particular pair I have here does not have this option. All of the internal
wiring is Furutech OF copper with a Teflon wrapper.

As you might expect, the woodwork being in Bamboo and having a
light finish is quite attractive. The overall build quality is first rate. This
pair of speakers requires a stand. With my low seating (Futons), I needed an
18Ē speaker stand. The ones I chose were solid wood spools (8Ē diameter
stanchion) that were originally designed to be plant stands. They work
exceptionally well and are pretty much resonant free, just as you want in a
stand.

Open Baffles or Dipoleís

I donít know if any of you have been paying attention to the
online forums over the past couple of years but there is a quiet revolution
taking place in the audio market. Besides the absolutely remarkable sounds
coming from computer based music servers, modified Squeezeboxes and inexpensive
DACís, the open baffle speaker is making huge inroads on the audio fringe.
There is a feeding frenzy going on right now by the DIY crowd over open baffles.
Some of the smaller speaker manufacturers are starting to take notice and are
now offering OBís in their product lines.

The open baffle or dipole speaker is nothing new. They have
been around since the early 1900ís. Over the years, a number of speaker
manufacturers have designed and marketed dipole speakers. Some of the more well
known dipole speakers are Magnepanís, Martin Loganís, Quadís, Carverís,
Innersoundís and Apogeeís.

If you arenít familiar with dipole speakers, essentially
what they are is an open backed driver that is mounted on a single piece of wood
(a baffle) without an enclosure around it. This driver radiates sound towards
you (in phase) and then is also allowed to radiate sound from the back side of
the driver (180 degrees out of phase). This out of phase sound is then reflected
off of the wall behind your speakers, which is then brought back into phase
because of the reflection (when sound bounces off of a flat rigid surface at a
right angle, the phase is reversed by 180 degrees). This ever so slight
delay in the reflected sound causes the sound waves (mostly in phase) to trick
your mind (psychoacoustically) into thinking that the recorded sounds have
greater depth and spatial qualities. It tends mimic the sound of a large
recording studio or hall. When done right, the sound can be utterly
breathtaking.

In recent years the DIY crowd (I consider myself part of that
group) has really latched onto the dipole/open baffle concept. With their modern
mentor Siegfried Linkwitz at the helm, they have created quite a stir on the
audio boards and the DIY speaker builder pages out in cyberspace. I myself am a
re-convert from the conventional monkey coffins. Straying at first from box
speakers to open baffles (ribbons) to back loaded horns (and front loaded
horns), Iíve finally settled on Lowthers in an open baffle using a boxed 15Ē
woofer to (actively) drive the bass. Iím here to tell you, there is nothing
like the sound of an open baffle speaker. It is completely free of all the
compromises associated with box speakers.

The ironic part of this adventure for me is, I popped my
audiophile cherry on an original pair of Maggieís way back in the early
70ís. Even back then I was absolutely taken with the concept of the dipole
speaker. Back then I made my own crude imitations of the dipole. Rather than
mounting the midrange to a baffle, I used to mount a tweeter on the back of my
DIY speakers to get some of those spatial cues and depth of image that the rear
reflections provide. Rudimentary as they were, they captured part of the essence
of a good dipole.

In more recent years I again played with dipole designs. I
used some 30Ē Carver ribbons to construct my own dipoles. I used them for a
number of years in a bi/tri-amped system until I found my musical soul mate, the
Lowther PM2Aís. I now use the Lowthers in an open baffle configuration.

Moving Forward

As usual, I tried these speakers out in each of my three
rooms, each using varying amplification. Feel free to click
hereand see the lowdown on each of my rooms including the size, layout and the
different gear used.

The Redrum

This is where the Sentinelís spend the most time. They
really seemed to mate with this room extremely well. The initial placement and
setup was truly a snap. I (literally) guesstimated where they would sound
decent, plopped them down and started listening. Iíd bet I didnít move them
more than about 6Ē in any direction to get them dialed in (though I did move
them around just to make sure this placement sounded the best). This really
reminded me of how the Roninís interacted with my friend Steveís room in
when they were in town last fall. There, we did the same thing, plopped them
down and started listening. They too werenít overly picky about placement at
all.

As it is with any open baffle speaker, you need to have them
out from your front wall at least three feet so that you get that great depth of
image. In my case and since Iíve got the room, I decided to pull them out
about four feet into the room. They are about two feet from the sidewall. This
position seemed to give me the best depth and most cohesive image in this room.

As I listen to the Sentinelís as driven by my big Radii tube
amps, I was definitely reminded of the sound I heard back last fall. The
Sentinelís project an exceptionally large and deep soundstage. Since these
speakers are coaxial and radiate in a 4pi manor, the sweet spot is quite
expansive. You can literally get up from your listening position and move around
in your room and still maintain a reasonable image. Sure, you might get slightly
more (or less) off axis information as you move around but the basic stereo
image stays remarkably stable. No Ďhead in a viseí sweet spot with these
speakers, that is very refreshing for a change.

Let us start with the audiophile stuff first this time, shall
we? As I listen to the Sentinelís ability to project a soundstage, I start
with Pink Floydís Signs of Life from Momentary Lapse of Reason.
The lapping water on the shoreline is coming from about four, almost five feet
from outside the speaker boundary. This is actually quite exceptional. Not many
speakers Iíve had in my little laboratory have been able to pull that one off.
I know Iím jumping ahead of myself but the first bass notes of this song are
quite deep, solid and impactful. I canít help but mention how well the
Sentinelís do in reproducing this quality of bass in this room.

As I listened for the placement of the performers on this
virtual stage, I played several tracks. The first was "Throwdown at the
Hoedown" from Left of Cool by
Bťla Fleck & The Flecktones.
Towards the end of the song, the recording engineer has Bela and Victor Wooten
panning back and forth between the left and right channels. The Sentinelís did
a fine job making that handoff without leaving a hole in the middle of the
soundstage. The next track I used was "Daraijin" from Mondo Head by
Kodo Drummers. In several parts of this song, there is a single row of over
ten drummers that stretches across the stage. The drummers trade individual
strings of beats, all the while peppering the image from far left to far right
with evenly spaced performers. The stereo mic used to record this picked up
those minor placement differences almost perfectly. In turn the Sentinelís
gave me those spatial cues that told me there was distance between each of the
drums rather than that one lump of sound coming from between the speakers.

Another thing that the Sentinelís excel at is soundstage
depth. When placed at a decent distance out from the head wall, the soundstage
extends about four to five feet behind the speakers and often deeper, providing
that is the way the music was mixed.

Since Iíve mentioned the bass reproduction of the
Sentinelís several times, letís go there next. Here I decided to try
something a bit different. I cued up Eric Serraís soundtrack to The Fifth
Element, the second track called Mondoshawan. If you remember from the movie,
this is the music that is playing when the spaceship lands next to the ancient
ruins at the beginning of the movie. This song has a synthed bass track that
goes down to 8Hz fairly evenly from about 200Hz. As Paul has mentioned on the
Nomad Audio website, you can expect that these speakers will do about 30Hz in
room. Well, I measured things a bit differently, at least in my room. On the
soundtrack, the Sentinelís died off at about 18Hz (-3dB, measured and verified
with my Sencore SP-295c). Between that point and the 200Hz, they were relatively
even with some minor boosts and dips due to room nodes and nulls (partially do
to my seating position). I reconfirmed this with several other musical tracks
including some test tones. Thatís pretty impressive for a sealed speaker.

The bass I heard coming from the Sentinelís was quite
impressive. It was very solid with very little overhang. I really think this is
attributable to the sealed box design of the woofer. On your typical vented
speaker, its ported tuning frequency (if not properly calculated) can cause all
kinds of weird harmonic overtones that make the speaker (woofer) sound slow and
undefined. This is not an issue with the Sentinel. Its bass is well defined and
quite deep in this medium sized room. The 8-inch Seas woofer sounds reasonably
large. It doesnít quite fill the room like a big 15-inch but they did a fine job
on nearly everything I threw at them.

When we move onto the ultra critical midrange, here the
Sentinelís faired quite well also. Playing both male and female vocals to
gauge, there was only a hint at the lower midrange being ever so slightly thin.
Some of my opinion might be due to the ultimate reference gear I use, Lowthers
and SETís. Nothing gets the midrange more Ďrightí than that combination.
My reference system is an extremely hard act to follow. Some of what Iím
hearing could attributable to this room and my seating position (most likely
culprit). Iíll know more when I take the Sentinelís and try them in my other
rooms.

Staying on the midrange frequencies, the mids are reasonably
transparent. As I listen to tracks like Eric Claptonís One Track Mind
from his Back Home release, this CD is very clean sounding. The Sentinelís
only show hints of not retrieving the lowest levels of details. As I
played something on the opposite end of the musical spectrum, Stravinskyís Rite
of Spring, Maazell and the Cleveland Orchestra, this piece revealed how open
sounding these speakers can be. The open baffle design transformed this piece
from a simple recording to a piece that gives the illusion of the concert hall.
The sound was big, open, dynamic and reasonably accurate.

The timbre of the speakers overall was quite good. They had no
problem convincing me that a piano was a piano and an oboe was an oboe. I
didnít notice much coloration at all. As it is with most rooms, speaker
placement and bass reproduction can be an issue. These speakers are no
different. If you get them too close to the room boundary, they can embellish
the bass notes. This really isnít much of an issue as a quick repositioning of
the speakers solves those issues. Although with rock and roll, moving these
speakers back a little bit made for some darned fun listening.

When it comes to the high frequency reproduction of the
Sentinelís, they did quite well also. The highs were smooth and extended
without being harsh or gritty at all. The handoff between the midrange driver
and the tweeter is relatively seamless, as I didnít notice anything of
consequence sonically. As you would expect from a nice sounding soft dome
tweeter, the high frequencies reasonably detailed without being overbearing or
sizzley. The highs came across smooth and accurate providing a nice amount of
Ďairí and transparency to the presentation especially when played with a
good vinyl rig.

Before I got out of the Redrum, I decided to try an amp that
seems to be an unlikely combination. I swapped out my high-powered (as far as
tube amps are concerned) modified Radiiís for the little factory Modified
JoLida 102b at somewhere near 25 watts per channel. Though slightly underpowered
in this room, I easily reached 90+dB peaks, which for some, is louder than they
normally listen.

I have to say without reservation, this combo sounded
extremely good, especially with vinyl. This little amp continues to amaze me.
Regardless of a speakers current draw demands and sensitivity, the little 102b
continues to do a fabulous job with all. Sure, you arenít going to rock your
brains out with 95dB but what you do get with this combination is
extremely good. The soundstage projected is even deeper and wider. As a little
bonus to this new combination, Iím actually starting to hear a bit of the
Ďpresenceí that I normally only hear in my SET/Lowther/Vintage Sub system.

The Grape Room

Next up is the smallest of my rooms. I had some serious
reservations about how these speakers would perform in this room. On numerous
occasions Iíve tried slightly larger speakers in here and nearly every time
they just didnít work without adding some really heavy acoustic treatments.
This time Iím hoping that since the Sentinelís are an open baffle, they will
load the room differently and work a little better with these room dimensions.

After I moved the Sentinels into place in this room and got
all of my wiring secured, I hit the play button on the latest Crosby Nash
release, Highlights. I did some very minor repositioning of the speakers (I
might have moved them 6Ē in a couple directions) and I had what was to be
their final position. After a bit of fiddling with the positions of room
treatments, I was ready to listen.

Upon restarting the CD, I was immediately taken by how well
they worked in this small room. From mids on up to the highs, the Sentinelís
performed nearly as well as they did in the Redrum. When it came to the bass, it
really wasnít as bad as I expected. Sure, I had some buildups (and a suck out
or two) as one would expect from a small room but they werenít nearly as bassy
as I had envisioned. A quick trip to my stock of mineral wool and I had myself a
makeshift bass traps that I promptly placed in the room in a LEDE configuration.
All of a sudden, the Sentinelís sounded quite even from top to bottom.

This confirmed my thoughts that these large open baffle
speakers would work fine in this room even though they appear to be too big at
first glance. The end result of these minor efforts gave me a system that
sounded very good. The bass was solid and deep without any major sonic
anomalies. The mids through the highs were clean, open and spacious without any
hints of congestion.

As I popped on some more familiar reference music, I found the
placement of the performers on discís like the Adam Rafferty Trioís release
Three Souls, to be as precise as when the Sentinels resided in a much larger
room. The other item of note is the stability of the image. Just as it was in
the Redrum, the image stayed remarkably firm as I repositioned myself on both
ends of the Futon. That said, as intimate as the confines of this room are, I
must say that the overall image projected is smaller than that when it was in a
larger room. That too is very reasonable expectation considering the small room
dimensions.

The Blue Room

This is a room that Iíve never had any issues getting a pair
of speakers to mate well with. The confines are very speaker friendly,
especially open baffles. Since the Sentinels are all of 84dB, I went the solid
state route. I brought the Odyssey Audio Khartago down from my Grape Room.
Having bipolarís for amplification and being very conservatively rated at
100wpc, I knew this should be a very good mating.

Just as I anticipated, these speakers fit this room like a
hand in glove. The extra power and current drive the Khartago let me really see
what these speakers are made of. As a tube fan, you might imagine Iíd have to
slide in some snide remark about solid state but you wonít find it here
(believe it or not). I did use my highly modified Korato KVP-20 tubed pre along
with my Paradisea tubed DAC in front of the Odyssey amp but the real credit for
the sound goes to Klaus Bunge for making a great sounding amp that can drive
just about anything.

As you can imagine in this room, these speakers were freed
from the constraints and confines of smaller rooms and lower powered amps. In
turn, the Sentinels were fed the current they tend to like. As I cranked the
volume ever upwards, the Sentinels opened up even more. As good as they sounded
with the high powered tubes, I have to admit that they performed even better
with the drive of solid-state.

There were some definite advantages to having them in a much
larger room. The soundstage got noticeably deeper and wider. At the same time
the dynamics seemed to improve a fair bit. The sound became more relaxed and
easier to listen to. I suspect that this is due in part to the way the
Sentinelís load a larger room. Music played at high SPLís seemed to have a
better overall flow in this much larger space.

The only drawback that I noticed to having the Sentinelís in
this considerably larger room was a roll off in the lowest octave of bass. That
shouldnít come as a surprise to anyone though. Unlike their big brother the
Roninís, these use a smaller 8-inch woofer in a sealed enclosure. When you move
them out from the headwall to get the deepest, widest soundstage, the lowest
octave of bass will drop off slightly. Thatís not a big issue if you donít
mind compromising and moving the Sentinelís back towards the wall to utilize
its gain. The other issue was loosing the midrange fullness of an all tubed
system. Again, this is a compromise. All of the great qualities of tubes that I
love relented to the sheer authority, raw power and much better overall control
of solid-state. This was a seriously good mating.

In The End

Before I finish up, let me wander for a bit. Through the years
Iíve had good exposure to dipoles of nearly all flavors. Some have been
extremely good, some have been just average. To date I think my favorites have
been either the Maggie 3.6ís or a pair of highly modified pair of IIIAís
that a good friend of mine owns. Iíve recently heard the original
prototypeís of the Martin Logan Monolithís that were pretty amazing. Iíve
always felt that an open baffle speaker has stood head and shoulders above the
vast majority Ďbox speakersí. They do Ďaudiophileí things that your
typical speaker could only wish it could do. Of all speakers, I think open
baffles are my favorite.

What I heard in each of my different sized rooms confirmed
that an open baffle speakers tends to be a fair amount more room friendly than
your typical box speaker. Sure, you will have bass issues to deal with in
smaller rooms but from the midrange up, they load a room in a way that is far
friendlier. This seems to make placement much more forgiving. Donít forget
that if you want a big, deep soundstage you will need to allow yourself some
distance from your headwall.

The Sentinelís give you a couple of advantages over your
conventional dipole speaker such as a Maggie. First, is sheer SPLís. These can
play considerably louder than a ribbon. At high SPLís ribbons start to
compress and slap. Also the bass from the Sentinelís (subjectively) has more
slam. Again, this is because of the limited excursion of a panel. On the
opposite side of the spectrum, the downside of the Sentinel over a ribbon is
transparency. Ribbons have a cleaner, slightly more accurate sound to them (to
my ears). The bass from a ribbon is significantly Ďtighterí sounding. In
turn, a ribbon tends to have a more Ďpresenceí in its presentation. Neither
of these are Ďrightí, neither are Ďwrongí, both are merely preferences
and tradeoffís in the way you want your music served up.

In my time with the Sentinelís I found that they do like a
bit of juice. They really start to come alive after about 90dB. Tubes worked
just fine when they were driving them (without the Zobel option) but I have to
admit they did come to life when I fed them with a solid-state power
plant. I can only imagine what they might have sounded like with a couple of
hundred watts of pure Class A power. Iíd bet good money they would perform
even better.

The Nomad Audio Sentinelís are quite the speaker in
my opinion. They might not do everything perfectly but at just over $2000, Iíd
much rather live with them than most any other speaker in that price range that
Iíve heard. If you get a chance, you really owe it to yourself to give them an
audition.

My Ratings

Please
keep in mind this rating system is used to compare the Nomad Audio Sentinelís
against absolute perfection, or a money no object speaker design. If you see
what you think may be a low(ish) score, itís because there are speaker designs
that are even more refined but consequently cost considerably more. To top that
off, if I assign 5ís across the board, Iíve just painted myself into a
corner leaving no room for that Ďultimateí speaker. You wonít see me
handing out many 5ís. In turn, I feel I need to leave room in the ratings
system to accommodate those speakers.